Method of preparing dental investments and the like



J.*J. CANTOR Sept. 4, 1951 METHOD OF PREPARING DENTAL INVESTMENTS ANDTHE LIKE Filed Jan. 15, .194e

INVEN TOR.

Patented Sept. 4, 1951 'This invention `relates to a method forpreparing dental investmentsrand similar molds.

It has long been recognizedinthe preparation of dental investments thatair bubbles'in th'ein- .vestrnent mix linterfere with the ,productiono'f castings which are Vin exact conformity with ythe `pattern andattemptsheretoforehave been made lto `eliminate these airbu'bblesvso'that vmore pre- `cise castings can be obtained.

Generally speaking, vtechniques endeavoring to eliminate such bubblesmay 'be divided into Vtwo classes. `In the first class, the plaster orsimilar self-hardening material used to form the investment or mold wasmixed with its liquid such as water and then subjected to vacuum `with'the intention of extracting from the mix all of the gases existingtherein vincluding air and water vapor. When the mix Was'thus preparedit was thenpoured in the flask about the vpattern and .allowed lto-set.An objection to this method resided in the .fact that Yin the vcourse ofpouring the mix afterithad'been evacuated `around the ,pattern ,them'ixusually had adequate opportunity to lpick upor absorb new air whichbecame `entrained therein and prevented or greatly retarded theabilityof securing.liighlyprecise cast- ,.ings. 'IJhecseoond methodconsisted of rst mixvving the plaster and subjecting it to vacuum,

,pouring it into the 'flaskaround the pattern and then "subjecting the.poured plaster to a second Vvacuum designed to extract the air from thepoured material. A generalrevie'w of these procedures is Lto be .found.in the article entitled Principles of Vacuum Investing by K. -W. Rayappearing in the DentaTLaboratory'Review for September 1946, page 27.

A.Both of the above outlined procedures are still subject to a number ofobjections. One such objection exists in the fact that when the 'plaster,and water have been mixed to form a paste or slurry the ability of theVacuum to draw outk the air bubbles from the paste or slurry is highlyrestricted in that the high viscosity of the mix retards the passage ofthe air bubbles therethrough. Furthermore, if the pattern is quiteIintricateit may form pockets in which the air bubbles maybe disposedand these are held from escape through the plaster by the patternitself. Another Iand more serious objection resides in -the fact 'thatafter the Water and dry plaster lhave'been mixed together the plasterimmediately begins to set. Consequently, the creation of 2 heretoforeemployed -'have :been largely 4restricted intheir use to'castings-ofsmall'size.

An object'of'the presentinvention is toprovide a method proceeding upona different *theoryfrom those heretofore `developed wherein the dryplaster is placed in 'a suitable `'flask around vthe kpattern andthenissubjected'to vacuum "to remove the air therefrom. As the plaster is dryor substantially so, the extraction of vthe air therethrough isaccomplished in amuch easier manner than endeavoring4 to 'extract `airbubbles through a moist paste vor slurry. AWhen the air 'has been thusextraote-dfrom thedryplasterand preferably while Yit lis still subjectto vacuum, wateror other liquid, which may y"or may-notbe but which ispreferably iree lof vair or oxygen, is admitted to the vacuum chamberyand allowed to wet the plaster 'causing it Ato commence to set. As soonas the'pl'aster has beenthorough'ly'we't ambient air 'pressure isallowed to enter v4the Vacuum chamber and ir Athere -are any remainingair bubbles or Vapor vbubbles `within the'investment these areimmediately compressed Yor collapsed by -the air pressure. The resultyobtained is 'an investment which'is remarkably L'free of pockets`produced'by air `and vapor bub'blesso `that highly precise castings"are 'obtainable -with the plaster canta-ke place at anytime andtherethe appended claims, reference is Ihad 'to 'the accompanying 'drawings'for van nillustrative embodiment ofthe invention, wherein:

Fig. l is la verticaljsection through the apparatus 'designed and usedin vaccordance 'with vthe `present invention;

Fig. 2 is a horizontal section taken substantiaily upon the line'Z--2upon'Fig. 1;'and

Fig 3 is a -partial view in vhorizontal Sectio-r1 taken substantiallyupon Vthe line 3-3 upon Fig. '1.

Referring to the accompanying drawings wherein similar 'referencecharacters designate similar parts throughout, the apparatusv'illustrated consists -oi' 'a base generally in rthe form of atransverse bar Ill on which is mounted a circular plate II. A suitablegasket such as rubber or rubber-like material indicated at I2 isrecessed in the plate II to form an air-tight seal with the bottom edgeof a bell jar I3. A ask I4 is adapted to be positioned on the basebeneath the bell jar and this flask has at the bottom thereof theconventional crater forming member I5 and the gate or sprue formingmember I5 on which the pattern I1 is positioned in the usual manner. Inaccordance with conventional dental practice the pattern is formed ofwax. On top of the flask I4 there is positioned a ring I8 which is helddown against the flask by means of an apertured plate I9. Studs extendupwardly from the base 2I that supports the flask and thumb nuts 22 arethreaded thereon enabling the plate I9 to be tightened downwardlyagainst the top of the flask so that the base, crater forming member,flask, ring, and plate I9 may be held together as a unit when theseparts have been assembled. The flask I4 has openings in the side wallsthereof some of which may be merely in the form of perforations 23 andothers of which indicated at 24 may be in the form of horizontal slots.These openings are covered externally by means of gauze or equivalentmaterial indicated at 25 which is retained in place in any suitablemanner such as, for example, by rubber bands or split rings indicated at26. 'Ihe bar I9 has a Water passage 21 formed therein that leads to thebase of an upstanding stud 29 that is mounted on the plate II. This studprovides an outlet 29 for water that can be admitted through a supplypipe 30 past a needle valve 3I. Within the belljar there is alsodisposed an upwardly extending tubular stud 32 the top of which is opento the interior of the bell jar and the bottom of which communicateswith a passage 33 that is adapted to be connected to a vacuum gauge 344and. to a conduit 35 that leads to a vacuum producing or creating means.The vacuum creating means may be any conventional apparatus for thispurpose such as a vacuum pump or a water aspirator and when operatedserves to create a high degree of vacuum within the bell jar I3. Collars36 and 31 are adjustably mounted on the two studs 28 and 32 such as bythumb screws 38. These collars may be tightened down against anapertured plate 39 which is positioned against the tops of the thumbnuts 22 to hold the entire flask assembly in place.

The method performed utilizing the above described apparatus issubstantially as follows. The pattern I1 is mounted on the sprueproviding member I6 in the conventional manner and the flask I4 with itsapplied gauze 25 and retaining elements 26 is positioned on the craterforming member I5. The ring I8 which may be regarded as an excessplaster retaining ring is then mounted on the ask and the entireassembly clamped together by means of the thumb nuts 22. This assemblycan then be positioned on the plate I I and clamped thereon by means ofthe collars 36 and 31. Dry plaster or other cementitious material whichbecomes self-hardening when mixed with a liquid such as water is thenpoured into the flask. As materials of this type usually contract onbeing moistened, an excess of the dry plaster is poured into the flaskso as to partially ll the ring I8 and thus compensate for shrinkage. Thebell jar I3 is thenapplied to its gasket I2 and the vacuum creatingmeans connected to the conduit 35 is then operated. During the creationof the vacuum Within the bell jar the valve 3| is maintained in closedposition. As the plaster or equivalent material within theV flask is drythe air contained therein can readily pass out through the intersticesbetween the dry particles and the vacuum can be retained in the bell jarI3 for an indefinite period of time to assure complete extraction of theair. When it is desired to form the investment or mold the needle valve3I is opened allowing water to enter from the conduit 30, pass theneedle valve and go through the passage 21. This water enters the belljar through the inlet 29 and is allowed to ll the bell jar up to a levelWhere it may wet the plaster within the flask through the perforations23 and/or 24. In many instances the plaster will wet itself throughoutby capillary action but if desired the vacuum chamber may be floodedwith water so that the investment is completely submerged. In thepreferred method the water or liquid used is characterized by the factthat it is free or substantially free of all dissolved air or oxygen andother objectionable gases. Such water may be water that has beenretained under a vacuum over a prolonged period of time or may be waterthat has been thoroughly boiled to drive out air and other gases. Insome instances the water may be from a supply containing dissolved airor oxygen and as this water is admitted to the vacuum chamber within thebell jar I3 very slowly it will have the dissolved air removed therefromon admittance. Where the Water or liquid is rendered gas free prior toadmittance the particular manner in which it is thus prepared isimmaterial in so far as the present invention is concerned. It is merelydesirable that the water that wets the plaster be substantially free ofair or gases so that it will not reintroduce air or gases into the dryplaster that has just been evacuated. In this way the dryair-freeplaster is wetted in situ around the pattern and while the Vacuum withinthe bell jar is maintained. When the plaster has been thoroughly wet theconduit 35 is quickly opened allowing air to enter the bell jar and thewater therein can be either drained off through the conduit 30 such asby siphoning or the bell jar may be lifted from the gasket I2 allowingthe water within the bell jar to ow off of the plate II. 'Ihe entranceof ambient air pressure into the chamber causes any water vapor bubblesor any remaining expanded air bubbles to be compressed into very smallvolume so that these do not interfere with the obtaining of a highlyprecise mold from the pattern. 'I'he flask can then be treated in anyconventional manner to remove the wax pattern I1 leaving a clean drymold cavity. l

In the course of removing the air from the dry plaster within the flaskit may be advisable to vibrate or successively jar the entire apparatusso as to disturb the plaster and thus facilitate egress of air therefromwhile the plaster ls in its dry condition. In most instances, however,the air will readily escape from the powderedv plaster without suchvibration or jarring primarily because egress of air therethrough is nothindered by the liquid. When the water is admitted to wet the driedair-free powder, the plaster usually contracts and in so doing the levelof the plaster within the flask I4 and the ring I8 may settle to a levelapproximately even with the top of the flask I 4. Y

In the above described manner a mold or ihvestment can be producedhaving high fidelity to the shape ofthe pattern and which when thevacuum could be created immediately after the plaster and Water had.been mixed together and the plaster started setting, no suchlimitations are presented herein. For this reason, much larger patternsmay be employed if desired or necessary than those capable of being usedwith prior processes above referred to.

Various changes may be made in the details of construction withoutdeparting from the spirit and scope of the invention as deiined by theappended claims.

I claim:

1. The method of preparing a dental investment or similar mold whichcomprises placing dry plaster in a flask about a pattern and evacuatingair from the same while dried and While evacuated wetting the plastercausing it to commence to set.

2. The method of preparing a dental invest- 3. The method of preparing adental investment or similar mold which includes bringing into contactaround a pattern dry plaster and water from which the air has beenremoved and after the plaster has been thus wetted allowing the plasterto set.

4. The method of preparing dental investments or similar molds whichincludes bringing into mutual contact dry plaster and Water around apattern while under a vacuum.

5. The method of preparing a dental investment or similar mold whichincludes bringing into mutual contact around a pattern plaster andgas-free Water While under a vacuum.

6. The method of preparing a dental investment or similar mold whichincludes as a step thereof placing dry plaster in a flask about apattern and evacuating the air from the same while the plaster is in adry state.

JACOB J. CANTOR.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 391,830 Zuber Oct. 30, 18881,152,964 Nielsen Sept. '7, 1915 1,585,348 Hck et a1. May 18, 19262,089,552 Harrison Aug, 10, 1937 2,249,890 Droge July 22, 1941 2,392,929Lee Jan. 15, 1946 2,414,093 Cole et al. Jan. 14, 1947 2,416,506 Vest eFeb. 25, 1947 2,490,193 Barr Dec. 6, 1949

